Oil and gas is recovered by drilling into a hydrocarbon-bearing formation, for which purpose a drill string terminated by a drill bit is used to form a wellbore. The drill string formed from a series of connected drill pipe stands is rotated to remove formation ahead of the drill bit. Drilling mud or other fluid is pumped through the drill string to cool the drill bit, and to aid the passage of drill cuttings from the base of the well to the surface, via an annulus formed between the drill string and the wall of the wellbore.
At fixed intervals, the drill bit is removed from the wellbore and a casing comprising lengths of tubular casing sections coupled together end-to-end is run into the drilled wellbore and cemented in place. A smaller dimension drill bit is then inserted through the cased wellbore, to drill through the formation below the cased portion, to thereby extend the depth of the well. A smaller diameter casing is then installed in the extended portion of the wellbore and also cemented in place. If required, a liner comprising similar tubular sections coupled together end-to-end may be installed in the well, fastened to and extending from the final casing section. The liner may, or may not be, slotted or perforated.
Another method of inserting successive tubulars into the well bore involves a method of coupling a tubular liner to another previously installed tubular member in a borehole by installing a tubular liner and an expansion device into a borehole such that the tubular liner is overlapped with an existing tubular member. A sealed portion of an interior region of the tubular liner is pressurised with injected fluid so that the expansion device may be displaced uphole by the fluid to progressively expand the circumference of the liner.
After a series of liners has been installed, drilling through to increase depth of well bore can be carried out.
Once the desired full depth has been achieved, the drill string is removed from the well and then a work string is run-in to clean the well. Once the well has been cleaned out, the walls of the tubular members forming the casing/liner are free of debris so that when screens, packers, gravel pack assemblies, liner hangers or other completion equipment is inserted into the well, an efficient seal can be achieved between these devices and the casing/liner wall. The well is then completed by locating a string of production tubing within the casing/liner, through which well fluids flow to surface.
Completion operations and production can be adversely affected if the casing or liner in the completed wellbore is imperfectly connected, or otherwise damaged. Accordingly pressure testing is normally carried out during installation of the casing or liner and it is essential therefore, to be able to conduct pressure testing selectively at any given depth in the cased/lined wellbore. The ability to repeatedly conduct such tests within the wellbore in a single trip of the workstring is desirable.
Frequently, downhole tools are actuated by means of introducing obturating means into the circulation fluid, such as a plug, ball or dart, said obturating means being carried in the circulating fluid into contact with a seat within the target tool, and thereby effect a restriction in fluid flow, and consequential pressure build-up as pumping of circulation fluid continues. Typically the increased pressure is used to overcome shear fastener retention means in the tool to release a functional component or otherwise reconfigure the tool from a passive mode to an active operational functionality. Thus for example a slidable sleeve within the tool may be displaced to reconfigure flow paths to provide a radial fluid jetting function. When the desired operation has been completed it may be desired to restore the tool to a passive, non-functioning mode, and to re-establish circulation of fluid. This may entail an additional shear step.
Whilst appropriate design of such tools reliant upon use of shear pins can be very effective, it is not unknown for unexpected operational performance, such as premature yielding of the shear pin due to say an unforeseen event such as jarring of the string within the wellbore.
Additionally, when the high pressure required to initiate a shearing event is released suddenly by yielding of the shear pin either at its design yield point or prematurely, two undesirable effects may occur. One consequence is the transmittal of hydraulic shock down the string which may cause damage to downhole equipment or trigger de-stabilisation of unprotected formation, or cause further tools below the intentionally actuated tool to be prematurely actuated. Another consequence arises from the additional fluid pumped into the workstring to create the pressure build up being suddenly released upon the shear event to surge down past the tool which may also have unpredictable and undesired effects.
It would be desirable to address the risks of a shear fastener failure due to unforeseen operational performance conditions.